This project proposes to develop novel nanofiller resins based on tantalum oxide and silica. Our hypothesis is that the desirable properties of current microfilled resins can be retained while avoiding their major shortcomings through the development of non- aggregated nanoparticle fillers. Our overall strategy and objectives are to synthesize Ta2O5 and SiO2 nanoparticles, with varying size ranges and surface chemistries, in order to optimize four properties of resin composites: mixing rheology viscosity at high filler volume loadings to obtain satisfactory working/handling properties, toughness to resist wear and attrition, polishability and transparency to provide esthetic characteristics, and radiopacity for diagnostic utility. The rationale is that optimization can be accomplished by synthesizing filler particles that have the following properties: 100 nm or less in size to provide high polishability and avoid light scattering (thus retaining translucency)1 10 nm or more in size in order to efficiently inhibit crack-propagation (thus, enhancing wear resistance), independent and non-network forming (thus limiting viscosity buildup and allowing a high enough volume loading to toughen the structure and substantially offset polymerization shrinkage), strong interaction and coupling with acrylic matrix monomers so as to form a single phase material (thus eliminating interfacial fracture and the resulting catastrophic failure modes), and inherent radiopacity with the ability to adjust x-ray absorbance (therefore eliminating the need for hydrolysis-prone heavy-metal glass fillers and enabling radiopacity to be matched to the diagnostic requirements of the clinical application). Nanofillers with the above properties constitute a significant advance in restorative materials, independent of the other projects within the program project. Together with liquid crystal monomers and/or a dentin-inducing cavity treatment, nanofillers will form one component in a superior restorative system which can become an integral part of the dental armentarium and a viable amalgam replacement.